In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionixed water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Referring to FIG. 1A, a conventional rotary-type CMP apparatus 50 includes a wafer carrier 52, a polishing pad 56, and a slurry delivery arm 54 positioned over the polishing pad 56. The wafer carrier 52 is mounted on the bottom end of a vertical shaft 53 which rotates and presses a semiconductor wafer 66, mounted on the bottom surface of the wafer carrier 52, against the upper surface 60 of the polishing pad 56 as the polishing pad 56 is rotated. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution 64 onto the upper surface 60 of the rotating polishing pad 56. As the wafer carrier 52 rotates the wafer 66 against the upper surface 60 of the polishing pad 56, the polishing slurry 64 dispensed thereon by the slurry delivery arm 54 travels with the rotating polishing pad 56 until the polishing slurry 64 moves between the wafer 66 and the polishing pad 56. Accordingly, the polishing slurry 64 substantially polishes or planarizes the surface of the wafer 66.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical linear CMP apparatus 10 is shown in FIG. 1B. The linear CMP apparatus 10 is utilized for polishing a semiconductor wafer 24, i.e., a silicon wafer in removing a film layer of either an insulating material or a conductive material from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.
As shown in FIG. 1B, the wafer 24 is mounted on a rotating wafer holder 18, which rotates at a predetermined speed. The major difference between the conventional linear CMP apparatus 10 and the predecessor rotary CMP apparatus 50 (FIG. 1A) is that a continuous, or endless, polishing belt 12 is utilized instead of a rotating polishing pad. The polishing belt 12 moves in a linear, rather than rotational, manner in respect to the rotational surface of the wafer 24. The linear polishing belt 12 is mounted in a continuous manner over rollers 14 driven by a motor (not shown) at a predetermined rotational speed. The rotational motion of the rollers 14 is transformed into a linear motion 26 in respect to the surface of the wafer 24. In the conventional linear CMP apparatus 10, one or more polishing pads 30 are adhesively joined to the continuous polishing belt 12 on its outer surface that faces the wafer 24. A polishing assembly 38 is thus formed by the continuous polishing belt 12 and the polishing pad or pads 30 glued or otherwise attached thereto.
During the CMP process, the wafer holder 18 is normally operated in a rotational mode such that a uniform polishing on the wafer 24 can be achieved. To further improve the uniformity of linear polishing, a support housing 32 is further utilized to provide support to a support platen (not shown) during a polishing process. The support platen provides a supporting platform for the underside of the continuous polishing belt 12 to ensure that the polishing pad 30 makes sufficient contact with the surface of the wafer 24 in order to achieve more uniform material removal from the surface layer of the wafer 24. Typically, the wafer holder 18 is pressed downwardly against the continuous polishing belt 12 and the polishing pad 30 at a predetermined force such that a suitable polishing rate on the surface of the wafer 24 can be obtained. Air pressure is typically further used to push the support platen upwardly against the polishing belt 12 which, in turn, pushes the polishing pad or pads 30 against the wafer 24. A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting the downward force on the wafer carrier 28, the upward air pressure against the support platen, and the linear speed 26 of the polishing pad 30. A slurry dispenser 20, having multiple, typically eleven, slurry dispensing nozzles 34, as shown in FIG. 1C, is further utilized to dispense a slurry solution 36 through the respective slurry dispensing nozzles 34 onto the polishing pad or pads 30. As further shown in FIG. 1C, the slurry dispensing nozzles 34 are typically disposed at a distance “D” of 30 mm.
For Cu CMP applications involving low-K IMD (intermetal dielectric) for planarization, interconnect and gap-fill at 0.13 μm and smaller device generations, both the rotary CMP apparatus and the linear CMP apparatus typically utilize a polishing slurry that contains little or no abrasive in order to prevent or minimize damage to the low-k IMDs. For that type of slurry, the within-wafer slurry distribution is of utmost importance in achieving optimal polishing uniformity among all regions on the wafer surface, particularly with regard to 300 mm-diameter wafers.
Referring again to FIG. 1A, the dispensing nozzles 62 of the slurry dispensing arm 54 of the conventional rotary-type CMP apparatus 50 typically dispense the polishing slurry 64, having little or no abrasive, onto the rotating polishing pad 56 in such a location that the polishing slurry 64 initially contacts the center region of the wafer 66 as the slurry 64 moves beneath the rotating wafer 66. This causes higher polishing rates at the center relative to the edge regions of the wafer 66, resulting in an uneven polishing profile across the surface of the wafer 66.
Referring again to FIG. 1B, the slurry dispenser 20 of the conventional linear CMP apparatus 10 typically includes about eleven of the slurry dispensing nozzles 34 that are spaced along the length of the slurry dispenser 20. Accordingly, higher polishing rates are achieved on those regions of the wafer 24 that initially contact the polishing slurry 36 on the polishing pads 30, relative to the other regions on the surface of the wafer 24. This results in an uneven polishing profile across the surface of the wafer 24. Accordingly, a new and improved method is needed for dispensing a polishing slurry on a polishing pad in such a position or positions on the polishing pad that polishing rates, and thus, polishing profiles, on the wafer surface are more uniform.
An object of the present invention is to provide a new and improved method for dispensing a polishing slurry onto a polishing pad during a chemical mechanical polishing process.
Another object of the present invention is to provide a new and improved method for enhancing the polishing rates and polishing profile on the surface of a wafer.
Still another object of the present invention is to provide a method for enhancing the polishing rates and profile on the surface of a wafer using a rotary-type chemical mechanical polisher.
Yet another object of the present invention is to provide a method for enhancing the polishing rates and profile on the surface of a wafer using a linear-type chemical mechanical polisher.
A still further object of the present invention is to provide a method for enhancing the within-wafer distribution of slurry applied to a wafer during a chemical mechanical polishing process using a rotary-type polisher or a linear-type polisher.
Yet another object of the present invention is to provide a method for providing a substantially uniform polishing profile on a wafer by chemical mechanical polishing.
Still another object of the present invention is to provide a chemical mechanical polishing method which is well-suited to achieving a substantially uniform polishing profile on a wafer using a polishing slurry having little or no abrasive.